Welcome - PR template enforcer

[joenash]

There is a pull request template that has three functions:

• Prompting students to provide the correct information.
• Providing instructions such as applying labels.
• As a test whether students are paying attention.

This template is frequently ignored: when the required sections aren't deleted, it's a sign the instructions aren't being followed. The template is our brown m&ms.

When they don't delete the required lines, we should close the pull request, and post a message reminding them to follow instructions.